1. Field of the Invention
This invention relates to power line filters, that is to say filters capable of attenuating or suppressing interference present on an electrical network or mains supply to reduce the effect of such interference on equipment powered by the supply via the filter.
2. Description of the Prior Art
Mains electrical supply networks generally have interference voltages superimposed on the supply voltage proper. Interference voltages, which can be of a variety of natures and can arise from a variety of sources, can disturb the operation of, or even cause damage to, equipment fed by the supply, in particular sensitive electronic equipment such as, for instance, computers, electronic weighing machines, office machinery and so forth.
The presence of such interference voltages or mains or network supplies is, of course, well known and it is known to suppress them by means of power line filters, namely filters through which the power can be fed to the equipment and which are operative to filter out at least some of the interference to which the equipment may be subjected. A disadvantage of such power line filters is that a particular filter has a particular filtration characteristic. However, the particular nature of the interference present on the supply voltage to a particular power line filter cannot with accuracy be predicted since it may vary from time to time and since it may not be possible to specify (or at least specify with precision) the operating environment of a particular filter in advance. The effectiveness of the performance of a filter is dependent also upon other factors, such as, for instance, source and load impedances, which also cannot accurately be predicted since they may vary from time to time. In view of all this, the known power line filters are subject to the disadvantage that the filtration characteristics of a particular filter in a particular situation may at any time be far from optimum.
An object of the invention is to overcome or at least alleviate the above-mentioned disadvantage of the prior art power line filters.